The separation of mixtures of chemical compounds into two or more fractions by selective adsorption using molecular size adsorbents is generally known. The separation is usually effected by what is known as a relative adsorptivity separation where one component or fraction is more strongly adsorbed than another, or, a size selective separation which takes advantage of the uniform diameters of pores of a given molecular sieve adsorbent to permit the adsorption of certain compounds or fractions while essentially excluding others. The separation of normal paraffins from isoparaffins has been found to be particularly adapted to size selective adsorptive separations and a number of processes have been proposed for this purpose. Some of the processes have been based on contacting the mixed hydrocarbon feed in the vapor phase with a molecular sieve having pore diameters of about 5 .ANG. to adsorb the straight chain hydrocarbon compounds, i.e., normal paraffins, followed by desorption of the straight chain compounds at a lower pressure or higher temperature usually with the aid of a purge gas. Other processes have used a displacement purge material which is sufficiently strongly adsorbed to exert a displacing action on the adsorbed straight chain compounds with little or no change in temperature or pressure. One process of particular interest operates under essentially isobaric and isothermal conditions and desorption is accomplished using a non-adsorbable purge gas instead of a strongly adsorbable displacement purge material. Such a process is defined in detail in U.S. Pat. No. 4,176,053, issued to Holcombe.
The above-identified patent discloses a process for separating normal paraffins from an admixture with non-normal paraffins by passing a feedstock mixture of same in the vapor state and at superatmospheric pressure periodically in sequence through each of at least four fixed beds of a system containing a zeolitic molecular sieve adsorbent having effective pore diameters of substantially 5 .ANG., each of said beds cylically undergoing the stages of;
(A-1) adsorption-fill, wherein the vapor in the bed void space consists principally of a non-adsorbable purge gas and the incoming feedstock forces the non-adsorbable purge gas from the bed void space out of the bed without substantial intermixing thereof with non-adsorbed feedstock fraction;
(A-2) adsorption, wherein the feedstock is cocurrently passed through the bed and the normal paraffin constituents of the feedstock are selectively adsorbed into the internal cavities of the crystalline adsorbent and the non-adsorbed constituents of the feedstock are removed from the bed as an effluent having a greatly reduced content of the normal paraffin feedstock constituents;
(D-1) void space purging, wherein the bed loaded with normal paraffins to the extent that the stoichiometric point of the mass transfer zone thereof has passed between 85 and 97% of the length of the bed and containing in the bed void space a mixture of normals and non-normals in essentially feedstock proportions, is purged countercurrently, with respect to the direction of adsorption stage (A-2), by passing through the bed a stream a non-adsorbable purge gas in sufficient quantity to remove the void space feedstock vapors; and
(D-2) purge desorption, wherein the selectively adsorbed feedstock normal paraffins are recovered as a product stream by passing a non-adsorbable purge gas countercurrently with respect to adsorption stage (A-2) through the bed until the major proportion of adsorbed normals has been desorbed and the bed void space vapors consist principally of non-adsorbable purge gas.
Adsorption processes such as described above have been proven to be commercially useful and have been integrated with isomerization processes to provide a highly isomerized product that is useful as a blending component in the production of motor fuels, particularly gasoline. Such processes are known in the art as "TIP", i.e., TOTAL ISOMERIZATION PROCESS, and are described, for example, in U.S. Pat. No. 4,210,771, issued to Holcombe, 4,709,116, issued to Zarchy et al. and 4,709,117, issued to Gray Jr. Other similar adsorption/isomerization processes such as described in U.S. Pat. No. 4,717,784, issued to Stem et al., disclose the adsorption of monomethyl paraffins in addition to normal paraffins.
Generally, the TIP process comprises passing a stream containing a mixture of normal and non-normal hydrocarbons into an isomerization reactor to catalytically isomerize at least a portion of the normals in the presence of hydrogen and a catalyst composition, which typically is a zeolitic molecular sieve with a hydrogenation component. Other catalyst compostions such as alumina-base catalysts have been used as well. Since the isomerization reaction is equilibrium limited, the amount of non-normal paraffins in the feed to the isomerization reactor is minimized. The effluent from the reactor typically contains as much as 20-50 wt. % normal paraffins. The reactor effluent is partially condensed to provide a hydrogen-containing overhead which is recirculated to a main purge gas loop and a hydrocarbon liquid fraction which is passed to an adsorbent bed such as described above where the normal paraffins are selectively adsorbed and the non-normal paraffins are passed out of the adsorber as an adsorption effluent and eventually an isomerate product. The normal paraffins are then desorbed from the bed using purge gas from the main purge gas loop. The fresh feedstream can either be initially supplied to the isomerization reactor or to an adsorbent bed depending on the feedstream composition and other factors known to those skilled in the art.
Despite the usefulness of the above-described adsorption and isomerization processes, improvements are sought. In particular, it has been customary to terminate the adsorption-fill step of the adsorption cycle prior to the breakthrough of hydrocarbons into the effluent stream, i.e., when the concentration of hydrogen has dropped below the level in the non-adsorbable purge gas, e.g., below about 90 mol. %. The effluent stream, known as A-1 effuent, is rich in hydrogen and is typically recycled as purge gas to an adsorbent bed undergoing purge desorption, i.e., D-2 step. When operated in such a manner, there is still hydrogen left in the void spaces of the adsorbent bed when the adsorption fill is terminated and the adsorption step, i.e., A-2 step, is commenced. This remaining void space hydrogen is then passed out of the adsorbent bed with the adsorption effluent.
In the naphtha isomerization process or TIP process according to Holcombe ('771) or in the separation process according to Holcombe ('053), the cycles of the operation are set such that the composition of the hydrogen reaches some minimal acceptable purity level. The A-1 step was completed before any degradation of the hydrogen purity from 100 mol. % hydrogen was observed. At the same time a layer of isoparaffins is formed behind the hydrogen layer and in front of the feed layer. In the second step, or A-2 step, the introduction of feed was continued in order to force the layer of isoparaffins from the bed. In order to maintain the 100 mol. % hydrogen purity of the A-1 effluent, it was necessary to have approximately 15 mol. % hydrogen in the A-2 effluent with the isoparaffin product. This leaves the adsorbent in the bed saturated with normal paraffins and the void spaces of the bed filled with the feed composition. During the D-1 step, hydrogen gradually displaces the normal paraffins from the adsorbent and forces the feed from the bed. In the next step, or D-2 step, the hydrogen flow is continued with the almost pure hydrogen produced during the A-1 step and the normals are withdrawn from the feed end of the bed. This material comprising predominantly hydrogen and normal paraffins becomes the feed to the isomerization reactor.
It has been found that the presence of large and varying amounts of hydrogen in the adsorption effluent, known as A-2 effluent, can be undesirable. For instance, as shown in the Drawing in both U.S. Pat. Nos. 4,176,053 and 4,210,771, the feedstream is heated by indirect heat exchange with the adsorption effluent stream. Since the adsorption effluent stream varies in composition and molecular weight throughout the adsorption cycle due to the varying presence of hydrogen, the heat content also varies. Such variations can lead to inefficient heat exchange. Moreover, the presence of hydrogen in the adsorption effluent stream can require the use of flash vessels, also shown in the Drawing of the above-identified patents, to make a phase separation and recover the hydrogen-containing vapor from the liquid product. The recovered vapor stream is typically recycled to a recycle gas compressor for further use as purge gas. The liquid product is typically passed to a stabilizer for removal of light ends, e.g., butane and lighter.
Other processes have been proposed which incorporate additional equipment to accommodate hydrogen in the adsorption effluent stream. For instance, U.S. Pat. No. 4,831,207, issued to O'Keefe et al., discloses a TIP process that incorporates an impurity removal system that is useful for preventing catalyst deactivation from sulfur or nitrogen compounds. The process utilizes a reactor system to convert the sulfur and nitrogen compounds to H.sub.2 S or NH.sub.3 when necessary and adsorbent beds to adsorb the H.sub.2 S or NH.sub.3. When processing a feedstream that contains sulfur compounds, the above-identified patent discloses that the hydrogen-containing adsorption effluent stream from the A-2 step can be employed as a desorbent for the sulfur-containing adsorber bed. The resulting adsorption effluent stream is then phase separated and the overhead therefrom contains H.sub.2 S and hydrogen. Since combining the H.sub.2 S-containing overhead with the main purge gas loop in the TIP system could lead to sulfur poisoning of the isomerization catalyst, an additional compressor is often required to separate the recycle gas loops. The heat exchanger which exchanges heat between the thermal swing adsorber effluent and the sour hydrogen loop was oversized to anticipate a wide variation in the molecular weight and the heat capacity of the adsorber effluent during the cycle. Furthermore, since many compressors require a relatively constant supply of gas at the inlet, a surge drum can additionally be required to steady the varying hydrogen flow. A process flow scheme illustrating an additional compressor and a surge drum is set forth in FIG. 3 of the above-identified patent, equipment Nos. 243 and 237, respectively.
Accordingly, processes are sought which can reduce the amount of hydrogen in the adsorption effluent, i.e., A-2 effluent, remove the need for two separate hydrogen circuits, and thereby improve its usefulness for heat exchange purposes and avoid the need for additional process equipment.